The invention relates to covered wires, which are varnish-bonded, and to a novel process for the impregnation of covered circular-section or profile-section wires, as well as of braided conductors.
The wires and braided conductors employed in this process are preferably composed of copper or aluminum.
The technique whereby copper conductors or aluminum conductors are covered with an insulating layer of fabric fibers has long been known, this covering operation being performed, in some instances, after the conductor has received an insulating coating of varnish, and being followed, in some instances, by an impregnating operation in which a varnish is used.
In this prior art technique, the most diverse insulating materials come to be employed, eg. cellulose paper, cotton yarn, silk yarn, acetate yarn, polyamide yarn, etc., which are effected by elevated temperatures to a greater or lesser degree, and which usually receive a thermally stable impregnation in order to improve their thermal properties. It is more favorable to employ glass-fiber yarn for the covering, since this material, on its own, already meets the most rigorous temperature requirements and, furthermore, possesses valuable electrical properties. However, the wetting ability of glass fiber and its capacity to absorb impregnating agents are poor. For these reasons, it is necessary to carry out the impregnating operation concurrently with the covering operation, in order, by this means, to achieve firm anchoring of the glass fiber to the wire, and to prevent the glass-fiber covering from lifting away from the wire during the process of introducing the conductors into the slots, and during the bending and twisting which are associated with this insertion process.
The impregnating varnish also has the function of embedding the thin, fragile glass fibers, so that they bake together to form a solid mass, and thus impart a smooth surface to the covered wire.
The dielectric properties are improved by the filling of the interspaces during impregnation. In order to improve these properties further, a conventional varnished wire can be employed instead of a bare wire.
In the past, very diverse types of varnish have been used as impregnating agents, these varnishes belonging to Insulation Classes B and F, while some varnishes also belong to Class H. However, due to the fact that glass fiber is resistant to elevated temperatures, the complete system can usually be assigned to Class H.
The varnish-bound thread-covered wires are customarily produced on conventional varnished-wire machines, which have been supplemented by one to two covering apparatuses, or, alternatively, on units which are similar to the commercially available horizontal and vertical machines for producing varnished wire. The excess varnish is wiped off, preferably by means of a rubber sponge.
Using machines of the types mentioned above, the following operations are possible in a process for the manufacture of varnish-bonded glass-fiber-covered wires:
(1) Application, in the varnish bath, of a varnish to the bare conductor PA1 (2) Partial curing of the film of varnish PA1 (3) 2nd application of varnish PA1 (4) Covering, with thread, into the film of wet varnish PA1 (5) 3rd application of varnish PA1 (6) Second covering operation, frequently performed in the opposite direction PA1 (7) 4th application of varnish PA1 (8) Curing of the insulating structure obtained after (1)-(7) PA1 (9) 5th application of varnish PA1 (10) Curing PA1 (11) Repeat operations (9) and (10) several times, to meet the requirements, such as the desired layer thickness, etc.
The multiplicity of operations indicate that such a process is extremely inconvenient.
With the growing requirement for electrical conductors possessing good mechanical and dielectric properties allied with high stability at elevated temperatures, for use in fields involving high mechanical stresses and severe thermal conditions, eg. in welding transformers or railroad traction-motors or for braided connections to electrical components subject to high temperatures, the demand for thread-covered wires also increases especially for glass-fiber-covered wires, since the glass fiber, on its own, immediately gives rise to a high degree of resistance to elevated temperatures.
The above-mentioned process steps are now not only very extravagant in terms of labor and time. The process also possesses further disadvantages, which originate, in particular, as a result of the processing of varnishes with high solvent contents.
The disadvantages are:
(a) Contamination of the environment with solvent-laden waste air, or, alternatively, the use of expensive catalytic combustion processes for destroying the solvents.
(b) Inadequate adhesion of the covering to the conductor material, since the thick film of varnish, as described in the process, is not cured to the optimum extent.
(c) The development of bubbles during the curing of the film of varnish, which contains solvents, the presence of bubbles resulting in poor electrical properties, eg. poor dielectric resistance, poor dielectric strength, and inadequate resilience of the insulating structure.